This application claims priority from Japanese patent application number 2000-126706, filed Apr. 26, 2000, which is hereby incorporated herein by reference in its entirety.
The present invention relates to a back light unit for use in a liquid crystal monitor or the like, a liquid crystal display, and a method for manufacturing a light guide plate.
Liquid crystal displays are widely spread as image display devices for personal computers and various other monitors. Liquid crystal displays of this kind generally comprise a back light, planar light source for illumination, on a rear surface of a liquid crystal display panel to apply light to a liquid crystal surface with a predetermined spread in a fashion providing a uniform brightness, thereby visualizing images formed on the liquid crystal surface.
Such a back light uses a fluorescent lamp with a hot or cool cathode as a light source and must irradiate the overall surface of the liquid crystal display panel with light from what is called a linear light source comprising such a fluorescent lamp. Thus, the back light conventionally uses two methods, that is, a direct-light type and a side light type (an edge light type). The direct-light type has the fluorescent lamp placed immediately below the liquid crystal display panel and a uniformity plate and a diffusion plate installed thereon. On the other hand, the side light type has the fluorescent lamp installed on two sides or one side of a light guide plate made of a transparent resin so that light incident on the light guide plate is directed toward a liquid crystal display panel surface by means of a reflection section formed on a rear surface of the light guide plate; the light is then diffused into a uniform planar light.
The side light type back light may be thinner than the direct-light type back light and is thus suitable for display devices for portable equipment such as notebook computers.
FIG. 13a shows an example of the side light type back light. As shown in this figure, the back light 1 comprises a lamp 2 comprising a fluorescent lamp and acting as a light source, and a light guide plate 3. The light guide plate 3 has an incoming surface 3a opposed to the lamp 2 and on which light from the lamp 2 is incident, an outgoing surface 3b that faces a liquid crystal display panel (not shown) and from which planar light is emitted, and an opposed surface 3c that is opposed to the outgoing surface 3b. 
The opposed surface 3c has, for example, dot-like printing (not shown) applied thereto as the reflection section for reflecting light. A reflection sheet 4 is disposed on the opposed surface 3c, and a diffusion sheet (whose functions are to diffuse the light and to suppress bright lines) 5, one or more prism sheets 6, a prism protective sheet 7, and the like are laminated on the outgoing surface 3b. Furthermore, a metal reflector 8 is provided on the opposed surface 3c of the light guide plate 3 to reflect light.
In the light guide plate 3, light from the lamp 2 which is incident from the incoming surface 3a essentially advances toward a side end surface on the other end side while being totally reflected in the interior of the light guide plate 3. When, however, the light impinges on the dot-like printing or the like applied to the opposed surface 3c, the total reflection is compromised due to diffused reflection. Due to the reflection sheet 4 on the opposed surface 3c of the light guide plate 3, the diffused-reflected light is emitted from the outgoing surface 3b of the light guide plate 3. The dot-like printing or the like applied to the opposed surface 3c has coarser dots near the incoming surface 3a and denser dots on the other end side, in order to allow light to be uniformly emitted from the outgoing surface 3b (that is, to obtain a uniform luminance distribution).
Although attempts have been made to make the emitted light uniform by applying the dot-like printing to the light guide plate 3 as described above, bright lines K may occur on a screen, particularly near the lamp 2, as shown in FIG. 14. The bright lines K occur when a series of high luminance portions each extending linearly in parallel with the lamp 2 alternate with a series of low luminance portions each extending linearly.
Attempts have been made to find the causes of the occurrence of the bright lines K, and several occurrence mechanisms have been found which are based on a structure near the lamp 2; that is, action has been taken to individual causes. In fact, however, the occurrence of the bright lines K has not completely been clarified yet and is still one of the problems to be solved by those skilled in the art.
It is thus an object of the present invention to provide a back light unit, a liquid display device, and a method for manufacturing a light guide plate in which the occurrence of the bright lines can be diminished.
Then, the inventors examined the bright line mechanisms to obtain the knowledge that an edge portion formed between the incoming surface 3a of the light guide plate 3 and the outgoing surface 3b or the opposed surface 3c is formed into shapes different from an originally designed one after molding, that is, manufacturing tolerance during molding contributes to the occurrence of the bright lines K.
That is, the light guide plate 3 is generally formed by injection-molding an acrylic resin or the like, and the inventors assume that after molding, an edge portion E is curled as if chamfered (the example in FIG. 13b) or is thinned (the example in FIG. 13c), compared to a predetermined designed angle. Such a phenomenon is assumed to occur if during the injection molding, a resin material cannot fill every corner of the die due to its insufficient filling capability or the like, resulting in xe2x80x9cshort mold.xe2x80x9d
Under this assumption, the inventors manufactured the light guide plate 3 having an intentionally chamfered portion C on the incoming surface 3a, as shown in FIGS. 15a-c, as a case corresponding to, for example, the example shown in FIG. 13b (the example with the shape likened to have been chamfered), and used this light guide plate 3 to carry out an optical path simulation. That is, if the edge portion E of the light guide plate 3 is formed into a predetermined shape that is subjected to no chamfering or the like, no bright line K occurs theoretically; thus, in this simulation, the chamfered position C was formed on the edge portion to determine whether or not the bright lines K occur.
Only the prism sheet 6 was disposed on the outgoing surface 3b of the light guide plate 3 in order to eliminate other factors of the occurrence of the bright lines K. As shown in FIG. 15b, the prism sheet 6 has recesses 6a and projections 6b alternately and continuously formed on a bottom surface thereof at pitches of 50 [xcexcm] and having substantially triangular cross section a vertex of which has an angle of 68 [xc2x0]. In addition, as shown in FIG. 15c, the chamfered portion C formed on the light guide plate 3 forms an angle of 45 [xc2x0] relative to the incoming surface 3a and the outgoing surface 3b and has a chamfer dimension C1, C2 of 0.1 [mm]. As shown in FIG. 15a, in respect of the lamps 2, eight point light sources are arranged at equal intervals on a circumference of an imaginary circle of diameter 2 mm drawn in the center of the light guide body.
Then, the lamps 2 were lighted under the above described conditions for simulation to observe the intensity of light on a front surface of the prism sheet 6. The surface of the prism sheet 6 was partitioned into 1000 portions between the incoming surface 3a of the light guide plate 3 and the opposite end, so that the intensity of light was measured for each partition.
FIG. 16 shows the results of observation of the first to 100th partitions relative to the incoming surface 3a of the light guide plate 3. The axis of abscissa indicates the position (distance) relative to the end side, while the axis of ordinate indicates the integrated value of the amount of light emitted at an angle between 80 and 100 [xc2x0] (the unit is lumen).
As is apparent from this figure, the results of the simulation show intense light at substantially equal pitches: 4.1 mm, 9.1 mm, 13.2 mm, and 18.2 mm. That is, the results of the simulation indicate that when the light guide plate 3 has the chamfered portion C formed therein, the bright lines K are observed on the outgoing surface 3b. 
Thus, the light guide plate 3 manufactured under the above described conditions was used to actually produce a liquid crystal display, which was then measured for the luminance on a screen. FIG. 17 shows the results of the measurements, where the axis of abscissa indicates the position (distance) on the screen relative to the lamp (2) side, while the axis of ordinate indicates the integrated value of the amount of light emitted.
As is apparent from the results of the measurements, a periodic occurrence of the bright lines K was observed in the actual liquid crystal display using the light guide plate 3 with the chamfered portion C formed therein.
As described above, the inventors have found that the bright lines are caused by a failure to accurately reproduce a shape of the edge portion of the light guide plate originally designed during injection molding, that is, what is called manufacturing tolerances.
A back light unit according to the present invention based on this knowledge comprises a light guide plate for guiding light incident from an incoming surface thereof to an outgoing surface thereof to emit it as planar light, a reflection sheet located on a surface of the light guide plate which is opposed to the outgoing surface, a diffusion sheet for diffusing the planar light from the outgoing surface of the light guide plate, and a prism sheet having a prism surface for partly returning the light emitted from the outgoing surface of the light guide plate, from the outgoing surface to an interior of the light guide plate for diffusion, wherein an edge portion formed between the incoming surface of the light guide plate and at least one of the outgoing surface and the opposite surface has a radius of curvature or a chamfer dimension of 140 xcexcm or smaller. Furthermore, as a specific configuration of the prism sheet, the prism sheet has the prism surface opposite to the light guide plate.
The light guide plate is preferably formed of an acrylic resin of a high light transmissivity which is represented, for example, by polymethylmethacrylate (a refractive index of 1.49 and a critical reflection angle of 42xc2x0) and specifically an acrylic-based monomer or comonomer.
The xe2x80x9cradius of curvaturexe2x80x9d or xe2x80x9cchamfer dimensionxe2x80x9d of the edge portion of the light guide plate is obtained by measuring the light guide plate on a cross section orthogonal with the incoming surface. Furthermore, the term xe2x80x9cchamferingxe2x80x9d refers to providing a corner between surfaces with inclination or roundness, and the chamfer dimension corresponds to the distance between a position where an extension of the incoming surface intersects an extension of the outgoing surface and a position on the incoming or outgoing surface which is chamfered (see the dimension C1, C2 in FIG. 6).
According to an aspect of the present invention, the edge portion is not positively subjected to chamfering in order to obtain the above described dimension. Thus, according to the spirits of the invention, once a transparent light guide plate has been molded or produced by means of predetermined machining after molding, the edge portion need not be formed of a surface having the same continuous radius of curvature or the chamfer dimension C1, C2 of the incoming and outgoing surfaces need not be totally the same. The radius of curvature and the chamfer dimension have only to be within a range such as that shown above.
According to an aspect of the present invention, by setting the radius of curvature or chamfer dimension of the edge portion of the light guide plate equal to 140 xcexcm or smaller, light reflection conditions are prevented from differing from originally designed ones at the edge portion of the light guide plate to allow light incident from the incoming surface to be uniformly emitted from the outgoing surface, thereby reducing the occurrence of the bright lines.
Furthermore, by combining this light guide plate with the diffusion sheet and the prism sheet, planar light from the light guide plate can be diffused to further effectively reduce the occurrence of the bright lines.
Alternatively, if the radius of curvature or chamfer dimension of the edge portion of the light guide plate is set equal to 10 xcexcm or more, when a reflector is installed in the light guide plate while assembling the back light using the light guide plate, the edge position can be prevented from being caught in the reflector.
Alternatively, an aspect of the present invention provides a back light unit for guiding light from a light source to apply it to a liquid crystal panel as planar light, the back light unit comprising a light guide plate including an incoming surface on which light from the light source opposed to the light guide plate is incident and which is formed of a machined surface, and an outgoing surface from which the light incident from the incoming surface is emitted as planar light, a sheet-shaped diffusion member having a diffusion member for diffusing light, and a sheet-shaped prism member having recesses and projections opposite to the light guide plate. In this manner, when the incoming surface of the light guide plate is formed of a machined surface obtained using a tool such as an end mill or a milling cutter, if during the molding of the light guide plate, xe2x80x9cshort moldxe2x80x9d or the like results in a manufacturing tolerance with respect to the original design to, for example, thin or curl the edge portion, then this can be corrected to obtain the designed shape. As a result, the light incident from the incoming surface can be uniformly emitted from the outgoing surface, thereby reducing the occurrence of the bright lines.
In addition, the back light unit according to the present invention having the incoming surface of the light guide plate formed of a machined surface can be characterized in that an edge portion between the incoming surface of the light guide plate and the outgoing surface and between the incoming surface of the light guide plate and a surface opposed to the outgoing surface has a radius of curvature or a chamfer dimension of 140 xcexcm or smaller.
According to another category of the present invention, the present invention can be considered to provide a liquid crystal display. The liquid crystal display according to an aspect of the present invention comprises a liquid crystal display panel for displaying images and a back light unit provided on a rear surface of the liquid crystal display panel for irradiating the liquid crystal display panel with light, and wherein the back light unit comprises a light source for applying light, a light guide plate having an incoming surface opposed to the light source and an outgoing surface facing the liquid crystal display panel and in which an edge portion between the incoming surface and the outgoing surface and between the incoming surface and a surface opposed to the outgoing surface has a radius of curvature or a chamfer dimension of 140 xcexcm or smaller, and a bright line occurrence restricting sheet for restricting occurrence of bright lines caused by planar light emitted from the outgoing surface of the light guide plate. Furthermore, in another embodiment, the incoming surface of the light guide plate is machined.
According to such a liquid crystal display, the edge portion has a radius of curvature or a chamfer dimension within a predetermined dimensional range, and the bright line occurrence restricting sheet allows the light incident from the incoming surface of the light guide plate to be uniformly output from the outgoing surface, thereby reducing bright lines that may occur on the screen of the liquid crystal display.
The bright line occurrence restricting sheet may comprise the diffusion sheet, the prism sheet, or the like as appropriate, or of course a combination of the diffusion plate and the prism sheet or a plurality of prism sheets.
Alternatively, the present invention can be considered to provide a method for manufacturing a light guide plate. That is, an aspect of the present invention provides a method for manufacturing a light guide plate including an incoming surface on which light emitted from a light source is incident, the method comprising obtaining a light-conductive material by means of injection molding and using machining means to machine an incoming surface of the light guide plate material in order to obtain the light guide plate. This method can also comprise that during the machining of the incoming surface, an edge portion located between the incoming surface and a surface adjacent to the incoming surface is machined to have a radius of curvature or a chamfer dimension of 140 xcexcm or smaller. The light guide plate obtained using this method enables the configuration of a back light unit or liquid crystal display that enables the light incident from the incoming surface of the light guide plate to be uniformly emitted from the outgoing surface.
The present invention can further be considered to provide a method for manufacturing a light guide plate. That is, an aspect of the present invention provides a method for manufacturing a light guide plate for emitting light incident from an incoming surface, from an outgoing surface as planar light, the method comprising a die formed such that an edge portion between the incoming surface and the outgoing surface and between the incoming surface and an opposed surface opposed to the outgoing surface has a radius of curvature or a chamfer dimension of 140 xcexcm or smaller, and in that the die is used to injection-mold the light guide plate. By using this die to injection-mold the light guide plate, the light guide plate formed has a radius of curvature or a chamfer dimension of 140 xcexcm or smaller in its edge portion. The light guide plate obtained using this method enables the configuration of a back light unit or liquid crystal display that enables the light incident from the incoming surface of the light guide plate to be uniformly emitted from the outgoing surface.